topic Re: imx8mp Embedded ISP Gamma encoding in i.MX Processors

imx8mp Embedded ISP Gamma encoding

malik_cisse — Tue, 28 May 2024 10:25:48 GMT

Hello,

Our images look good, but I don't quite understand a factual situation regarding the gamma. The ISP seems to be applying gamma correction even though it is explicitly turned off.

I don't quite understand where the gamma encoding should come from. Maybe you can help me here.

The following points are known:

The image sensor (Sony IMX477) delivers linear image data.
Gamma decoding in the ISP (degamma) is programmed to pass-through (see XML): input array == output array.
Gamma encoding in the ISP is turned off (see XML: gc enable="false").
The ISP-CCM was set to the identity matrix and AWB was turned off. Manual WB was set to 1,1,1,1 (pass-through).
Gamma decoding of 2.2 is applied in the LCD display controller (implemented in the kernel driver).

The black values are correctly reproduced both from the LCD live images and from the YUYV snapshots (directly after the embedded ISP), which means that a gamma must have been set in the ISP.

However, since this is turned off, I don't understand where the gamma comes from?

Thx, M.Cisse

Re: imx8mp Embedded ISP Gamma encoding

Bio_TICFSL — Tue, 28 May 2024 14:33:46 GMT

Hello,

No gamma encoding is supported by ISP you better check your camera is causing the gamma encoding.

https://www.nxp.com/docs/en/application-note/AN12060.pdf

Regards

Re: imx8mp Embedded ISP Gamma encoding

malik_cisse — Tue, 28 May 2024 14:59:17 GMT

Hi,
Thank you for the quick feedback.

>>No gamma encoding is supported by ISP you better check your camera is causing the gamma encoding.
This statement might be correct for the software software ISP (SW-ISP). I am referring to the i.MX8M-Plus embedded ISP which definitely has deGamma (or sensor linearization) as well as Gamma encoding (also referred in the xml ISP config file as "gc enable").

The whole point using the i.MX8M-Plus is to avoid the SW-ISP.
But thank you for trying to help.
Cheers